A white-rumped bumblebee that has been in steep decline across its native range in the western United States and Canada appears to be making a comeback on the Colorado Front Range.

A survey of bumblebee populations carried out largely by University of Colorado Boulder undergraduates in undisturbed patches of prairieland and in mountain meadows above campus has turned up more than 20 rare western bumblebees, known scientifically as Bombus occidentalis.

This is the fourth summer of a planned five-year survey in Boulder County, led by biologists Carol Kearns and Diana Oliveras, both of whom teach in CU-Boulder’s Baker Residential Academic Program. The survey team, which this summer included five undergraduates along with Oliveras and Kearns, has been hunting bumblebees at nine different locations spanning low, middle and high elevations.

The first western bumblebee was netted last year at one of the low-elevation plots, located at around 5,000 feet. The same plot also was visited frequently by Kearns and Oliveras during a more general survey of all pollinators between 2001 and 2005.

“For five years we sampled fairly intensely at this one site and never found anything,” Oliveras said. “Then all of a sudden, last year, we found several bees at that one site.”

The surveyors also found western bumblebees last year at a mid-elevation site of around 8,000 feet. In all, the team found nine western bumblebees in 2012: three queens and six workers.

Because insect populations are notoriously variable from year to year, Kearns and Oliveras wanted to find the bumblebees for a second year before announcing that the western bumblebee appeared to be returning to the Front Range. This year, the team has netted more than a dozen western bumblebees at four different locations, including the same low-elevation prairie plot and all three mid-elevation meadows. The distance between the sites means that the bumblebees are likely from separate colonies.

“These are sites that are fairly far away from each other, even as the crow flies,” Oliveras said. “Within a plot, if you’re going to be conservative, you can say that all the Bombus occidentalis arose from a single colony. But between plots, that’s quite a distance for them. They wouldn’t normally be traveling that far.”

The western bumblebee was once ubiquitous across the western portion of the United States and Canada, Oliveras and Kearns said. Its northern range encompassed all of Alaska, the Yukon Territory, British Columbia and western Alberta. Its southern boundaries extended as far south as Arizona and New Mexico. The bumblebee’s range also stretched from the Pacific Ocean eastward through North and South Dakota, Nebraska and Colorado. But beginning in the late 1990s, the western bumblebee became harder and harder to find.

“They have been disappearing rapidly across the West Coast, and there have been only occasional sightings in the Rocky Mountains,” Kearns said. “People have found a few bumblebees on the Western Slope of Colorado, but we were looking for them here and we weren’t finding any.”

Several factors have been implicated in the decline of the western bumblebee, according to Kearns and Oliveras. The biggest suspect is a non-native gut parasite that may have been transmitted from commercially raised bumblebee colonies. While parasites and other diseases can kill bees outright, anything that affects the bumblebees’ food supply or nesting sites also will affect their ability to survive. That means that habitat loss, pesticides, climate change and invasive plants and animals may be contributing to the losses in western bumblebee populations.

Earlier this summer, reports that the western bumblebee had been spotted in the Seattle area were confirmed by local biologists, indicating that the bumblebees could be making a broader comeback.

The wider goal of the ongoing bumblebee survey in Boulder County is to catalog all the types of bumblebees buzzing around the area and their population size. The team has catalogued a number of different species during the last four summers, including the mountain bumblebee, the Nevada bumblebee, the two-form bumblebee and the central bumblebee, among others.

“Our whole interest in bumblebees relates to the fact that pollinators are declining, but there is no abundance data for bumblebees in this area from the past,” Kearns said. “How do you tell if something is declining if there are no abundance data? So we decided we’d get out there and we’d find out what bumblebees are here and how many.”

Each year, Kearns and Oliveras have recruited undergraduate students to help them. This summer, the undergraduate researchers were Benjamin Bruffey, Sam Canter, Sarah Niemeyer, Zoe Praggastis and Cole Steinmetz.

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“For five years we sampled fairly intensely at this one site and never found anything,” said CU-Boulder biologist Diana Oliveras. “Then all of a sudden, last year, we found several bees at that one site.”